Semi-conductor switches, such as power field effect transistors (MOSFETS), are widely used in prior art switched power supplies. Extra auxiliary transformer windings are usually required in such prior art power supplies to assure proper operation of the MOSFET power switches. However, this adds substantial complexity to the auxiliary supply system, especially if negative bias for speeding up the turn-off speed of the MOSFETS is required, and when more than one group of MOSFETS with different source potentials are used. Also, there are no means provided in such prior auxiliary supply systems for forcing the groups of MOSFETS to share the turn-off voltage when the groups are connected in series.
A more recent prior art power supply system uses snubber circuitry in conjunction with the MOSFET power switches to charge the positive gate bias for the MOSFET power switches during each cycle when the MOSFETS are turned off. As is well known, snubbers are normally used to reduce turn-off stresses of controlled semi-conductor switches. The snubber usually includes a capacitor in parallel with the semi-conductor switch to reduce the rate of rise of voltage across the switch when the switch is turned off, absorbing energy that would have had to be dissipated by the switch. Conventional snubber circuitry introduces losses since the charge on the capacitor is dissipated through a resistor to prepare for the next switch turn-off. The semi-conductor switch carries the discharge current of the capacitor when the switch is on.
The latter power supply which makes use of snubber circuitry avoids the need for the extra auxiliary transformer windings. However, it does not provide the negative bias needed to speed up turn-on of the MOSFETS, nor does it provide a means to force the MOSFETS to share the turn-off voltage.
Accordingly, an objective of the present invention is to provide an improved and simple circuit for producing gate drive bias for the MOSFET power switches which forces the MOSFETS to share the turn-off voltage when they are connected in series.
Another objective of the invention is to provide such an improved circuit which generates a negative gate bias for the MOSFETS in order to speed up switch-off of the MOSFETS.